1. Field of the Invention
The present invention relates generally to improvements in document storage, retrieval and transmission. More particularly, the present invention relates to improvements in compressing electronic images of documents.
2. Description of Prior Art
It is often desirable to store documents such as, for example, checks or other financial instruments in a bank, or public records, deeds, court records and the like in a government office. In many of these high-volume applications, the actual documents are stored. However, physical document storage typically involves substantial storage facility and manpower expenses. Because each document must be physically delivered to or located within a particular file, the storage and retrieval process is slow and documents are often improperly filed or mislaid.
One solution to the physical document storage problem is to store electronic images of the documents rather than the actual documents. This approach at present typically involves scanning the entire document to generate an electronic image, which may then be stored in a database memory location. The electronic image may be retrieved within a fraction of the time required in physical storage facilities. In addition, copies of the document image may be readily transmitted over data communication links for use in other locations, without physically removing the document from its file in order to, for example, make photocopies. However, a significant problem with this approach is the size, measured in bits, of the electronic image. In general, electronic document images include a large number of bits, and therefore require substantial storage and transmission capacity. Even when using known coding techniques to compress the image bits, such as the CCITT Group-3 and Group-4 image coding standards for facsimile machines, the total image size is still excessive. For example, assuming a facsimile scanning resolution of about 100 to 200 dots/inch, an electronic image of a single page of type-written, double-spaced text will include about 40,000 bytes, or 320,000 bits, in a 1-dimensional Group-3 format, or about 30,000 bytes, or 240,000 bits, in a Group-4 2-dimensional format. In many high-volume applications, the cost of the additional capacity to store or transmit electronic images may well outweigh the costs associated with physical document storage and retrieval.
A presently available technique for reducing image size involves masking off certain portions of the document prior to generating the electronic image. For example, the scanner may be directed to ignore the masked portions when the document is scanned to generate the image. Alternatively, a coding scheme, such as Group-3 or Group-4 coding, could be used such that the masked portion, which contains a plurality of identical pixels, is compressed into a small amount of memory. However, this approach has a number of drawbacks. For example, the masked portions generally must be in a predetermined location which does not vary from document to document. Masking therefore does not reduce memory requirements in documents in which the location of the desired information is unknown. Furthermore, all the information within the unmasked portions is typically stored, even though it may be unnecessary background detail, such as a pattern or illustration on a check. The background may also interfere with or obscure the desired information.
Another technique for compressing documents is disclosed in U.S. Pat. No. 5,182,656, issued to Chevion et al., and entitled "Method for Compressing and Decompressing Forms by Means of Very Large Symbol Matching" (hereinafter "Chevion"). The Chevion technique involves generating electronic images of an empty form and a filled-in form. A compressed image of the information added to the form is obtained by subtracting the empty form image from the filled-in form image. However, the Chevion technique requires very precise registration, or alignment, of the empty and filled-in form images prior to their subtraction, and is therefore computation-intensive. See Chevion, col. 5, lines 39 to 48. In addition, the Chevion technique is not well suited to distinguishing, for example, handwritten information which overlies or obscures portions of the form, as is often the case in documents such as bank checks.
Current image compression techniques typically do not discriminate between different portions of the compressed image. Therefore, if additional image processing such as automatic character recognition is desired, the compressed image must first be segmented to separate portions suitable for character recognition from portions which contain graphic or form-related information. Other problems with existing document compression techniques are described in, for example, M. Kamel et al., "Extraction of Binary Character/Graphics Images from Grayscale Document Images," CVGIP: Graphical Models and Image Processing, Vol. 55, No. 3, pp. 203-217, May 1993, which is incorporated by reference herein.
As is apparent from the above, a need exists for an efficient document image compression technique, which reduces storage and transmission capacity requirements and avoids the information location and isolation problems of the prior art.